Lubricating oil compositions used in an engine crankcase generally contain a zinc dialkyldithiophosphate additive to reduce engine wear. Unfortunately, it has been found that this use imparts certain phosphorus combustion products to the engine exhaust. These products can deactivate the exhaust gas catalyst used in most cars. Therefore, there is a need to reduce the amount of zinc dialkyldithiophosphate additive in engine oil but still retain adequate wear protection.
British Pat. No. 1,468,264 describes sulfurized additives made by reacting dialkyldithiocarbamate salts with relatively low molecular weight chlorinated polybutenes. Such additives do not offer any improvement in viscosity index.
Viscosity index is the ability of an oil to resist excessive loss of viscosity at elevated temperatures while still not being too viscous at lower temperatures. To improve this property, viscosity index improvers are conventionally added to motor oil. One class of viscosity index improvers are the olefin copolymers, especially copolymers of ethylene, propylene and a diene such as 1,4-hexadiene and/or vinylnorbornene.